ATMOSPHERIC NO3 .5. OFF-AXIS MEASUREMENTS AT SUNRISE - ESTIMATES OF TROPOSPHERIC NO3 AT 40-DEGREES-N

The sunrise column abundance of NO3 has been measured at Fritz Peak, C olorado (40 degrees N, 105 degrees W), using visible absorption spectr oscopy with scattered sky light as the light source. Measurements were made in the ''off-axis'' configuration, viewing the sky near the hori zon. This is a convenient alternative to direct Moon measurements, and the tropospheric component of the total NO3 column can be measured wi th comparable signal-to-noise levels to the direct Moon data. The aver age tropospheric NO3 vertical column measured during August-October 19 93 was about 4 x 10(13) molecule cm(-2). Assuming a layer 1 km thick i n the lowest part of the troposphere, this corresponds to 4 x 10(8) mo lecule cm(-3), or about 20 pptv. Under these conditions, the nighttime oxidation of isoprene by NO3 is comparable to the daytime oxidation b y OH.